Nanoparticles in an aqueous host

The behaviour of natural nanoparticles is far from understood when incorporated in a liquid, solid amorphous, or solid crystalline (mineral) host. Very few studies have addressed the effects of nanoparticle-host interactions on the stability of nanoparticles in the environment. In one of these studies, a water-driven structural modification independent of particle size was documented for ZnS nanoparticles at room temperature, confirming a significant role for the surrounding water molecules (Zhang et al., 2003). In the cited study, nanocrystalline particles of ZnS (average ~3 nm in diameter) were synthesized in an anhydrous methanol solution, and the nanoparticle-bearing solution was later transferred to vacuum at 25 C. After the rapid evaporation of methanol in vacuum, XRD measurements did not show any structural change of the ZnS nanoparticles, although EXAFS measurements showed that the methanol was retained on the surface of the particles. After the system was heated to 50 C to induce the thermal desorption of methanol from the nanoparticles, EXAFS data indicated that the ZnS nanoparticles transformed from a distorted structure (with methanol) to an unidentified dry phase. The original structure was regained after the addition of methanol, revealing a reversible structural transition associated with methanol desorption and re-adsorption (X-ray scattering data indicate a highly disordered structure of ZnS nanoparticles in methanol). When water was added to the system (50 µl H2O/ml methanol), UV-absorption measurements did not show any change in nanoparticle size, although a structural change was observed using XRD, suggesting that the disordered ZnS structure undergoes a transformation to a more sphalerite-like structure. Molecular dynamics simulations of model ZnS nanoparticles (2-5 nm) with different water coverages indicate that interactions between water and ZnS decrease the interfacial energy, resulting in an increase of crystallinity that propagates through the nanoparticle.

The findings reported in Zhang et al (2003) and summarized above strongly suggest that the structure and reactivity of nanoparticles, when surrounded by a liquid host, will depend on both particle size and the nature of the surrounding molecules. Therefore, the occurrence and prevalence of natural nanoparticles in aqueous environments (e.g., surface/ground/connate waters, and geothermal/hydrothermal solutions) may be controlled partly by nanoparticle-host interactions. Saunders (1990) documented evidence of aqueous transport of colloidal gold at the Sleeper deposit in Nevada. Gold and silica precipitated initially as colloidal particles (nanoparticles) at deeper levels in the hydrothermal system and were mechanically transported upward by the mineralizing solution, where they later coagulated due to cooling or boiling. The stabilization of nanoparticles in aqueous solutions by means of, e.g., water-driven structural transformations may be a key factor to explain the occurrence and transport of colloidal phases in water-saturated natural environments.

Nanoparticle-host interfaces in different dimensions

So far, we have seen how nanoparticles can have different properties than their corresponding bulk material, e.g., the lowering of the melting point as a function of size. However, the simplified description of a nanoparticle as a droplet whose surface tension changes as a function of radius, and thus, of curvature, is insufficient in most real-world cases. Most nanoparticles, even most atmospheric ones, are embedded in some kind of host matrix. This surrounding matrix can be other minerals, or, in the case of biomineralization, an organic matrix. Since this surrounding matrix is in contact with the nanoparticle, the host-nanoparticle interaction can stabilize or destabilize the nanoparticle. This process may also be size-dependent thus favouring certain size fractions. By far the most important parameter in this context is the interface energy. In some cases, we are able to calculate this interface energy or use indirect observations to measure it such as relative exposure of a particular face. Unfortunately, so far, the interface energy between nanoparticles and mineral hosts or organic templates has only been determined in rare cases, some of which will be discussed in this chapter. However, sometimes the different interface energy contributions can be estimated, which will be discussed along with some of the processes that lead to interface energy changes.

The most obvious contribution is the Coulomb or electrostatic energy. In order to obtain a highly attractive (i.e. negative) electrostatic energy, one possibility is that the nanoparticle surface and the host have different charges or are polarized in opposite ways. A charged mineral surface can be found where the mineral has been cleaved or grown along planes that have either positive or negative species exposed on that particular surface. Examples are calcite (001) surfaces with either just Ca2+ or CO32- ions exposed or the (111) surfaces of minerals with rocksalt structure. Cleavage planes tend to minimize surface charge; thus, we would only find them in nature if charge is compensated by particles in solution or if there is a solid counterpart of opposite charge (thus, we would also not find them in mineralogy textbooks listed as cleavage planes or growth surfaces; for more information on the thermodynamics of surfaces and interfaces, see, e.g., Al-Abadleh & Grassian, 2003; Butt et al., 2004). However, in contact with a surface of the opposite charge, such interfaces can be important in nature, especially in biomineralization processes. The functional groups of the organic matrix are often negatively charged, requiring a positive charge of the carbonate, phosphate, or oxide biomineral surface.

Interface interaction does not only have to take place by means of fixed charges. In contrast, below, we will describe polarizable metal nanoclusters on sulphide surfaces. This system is a good example for an interface, in which the charge can be shifted easily due to the conducting or at least semiconducting properties of these surfaces. Thus, nanoparticle and mineral host can electrically polarize each other which leads to a positive attraction.

More common than surfaces with opposite charges are surfaces with alternating charges that have to be matched by alternating and opposite charges of the other side of the interface. Using this principle, it becomes obvious that epitaxial matching is necessary. In order to fulfil this requirement, atomic separations (distances and angles) and often surface symmetry have to match on both sides of the interface. This is often difficult to obtain for nanoparticles in a host. Due to its small size, the nanoparticle changes its Miller indices of its surface planes at a small scale and so does the host around it. In addition, the atomic structure of both host and nanoparticle are difficult to evaluate because they are both likely to be distorted due to the narrow curvature of the interface. The Coulomb force is not the only contribution to the interface energy; other types of interactions are the atomic repulsion of atoms at small distances from each other, van-der-Waals forces, and spin-spin interactions. Finally, we need to take into account that interfaces are not static. Especially at elevated temperatures, we will find that the size, shape, and stability of nanoparticles are due to a complicated interplay of nanoparticle size, structure, and chemical composition of the host.

As a first example for nanoparticle formation on mineral surfaces, the formation of Fe(oxy-hydr)oxide clusters can be observed that preferentially adsorb to and grow from the corner of triangular terraces on monoclinic pyrrhotite (Fe7S8). In most adsorption/redox/cluster formation processes on sulphides, the semiconducting properties of these oxides and sulphides are important because these reactions are promoted by the co-reactivity over some distance, which we call the proximity effect (Becker et al., 2001; Rosso & Becker, 2003). Thus, the atoms of an element that forms a nanoparticle will preferentially adsorb in the vicinity of another element that catalytically enhances this adsorption process. It is important to note that on pristine pyrrhotite surfaces, only S atoms are exposed, visible as bright spots in Fig. 3Fig. 1 (Becker et al., 1997a). Fe atoms that are initially attacked by oxygen are only exposed along steps and, more prominently, where two steps join to form a corner of a terrace. In the example of the very initial oxidation of pyrrhotite (001) faces, after exposure to 6000 L O2 (1 Langmuir is the exposure to a gas at 10-6 Torr for 1s; in this case, exposure to O2 was at 10-4 Torr for 1 min at room temperature, which is equivalent to exposure to atmospheric O2 pressure for less than 1 ms!), Fe(oxy-hydr)oxide cluster formation is observed using scanning tunnelling microscopy (STM). Subsequent oxygen molecules are attracted to the already formed cluster due to proximity effect. Evidence for this is that no observable changes take place on flat terraces in this initial stage of oxidation and cluster formation. However, adsorptive structures form at corners of terraces or, where such structures already existed, adsorbates tended to increase in size.

For the formation of these nanoclusters, one species (Fe) is provided by the mineral, in this case pyrrhotite, the other (O, and, to a minor degree, H, from some water vapour) from the outside. Interestingly, Fe(II) in a pyrite surface changes its oxidation state before S does. This is in contrast to the typical macroscopic way of writing the chemical equilibrium reaction for sulphide oxidation to sulphate and Fe2+ in solution. In fact, charge transfer from Fe2+ on Fe sulphide surfaces to molecular oxygen is easier than from S because there is no kinetic inhibition to transfer spin density from O2 to the Fe on the surface (Becker et al., 1997a, 2001). In a subsequent electron transfer process, the oxidized Fe may get its electron back from a sulphide ion.